Correlations between nutrient concentrations and zooplankton populations in a mesotrophic reservoir

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1 Freshwater Biology (2002) 47, Correlations between nutrient concentrations and zooplankton populations in a mesotrophic reservoir J. M. CONDE-PORCUNA,* E. RAMOS-RODRÍGUEZ and C. PÉREZ-MARTÍNEZ* *Institute of Water Research, University of Granada, Granada, Spain Department of Animal Biology and Ecology, Faculty of Sciences, University of Granada, Granada, Spain SUMMARY 1. A year-round study was conducted in a mesotrophic reservoir to determine the dynamics of zooplankton populations as a function of food availability (edible phytoplankton), nutrient concentration, temperature and hydraulic regime. 2. Rotifer biomass was correlated with soluble reactive phosphorus (SRP) concentration. The abundance of the rotifers Keratella cochlearis and Anuraeopsis fissa were not correlated with food availability (measured by chlorophyll and cell counts) but showed a strong dependence on P availability. Another rotifer, Synchaeta oblonga, and crustacean species were not related to nutrient availability but seemed to be dependent on food concentrations, especially of some phytoplankton taxa. 3. In this field study, rotifers seemed more susceptible than Daphnia or copepods to P-limitation. Among rotifer species, Keratella seemed to be more susceptible than Anuraeopsis to P limitation. Different susceptibilities of zooplankton species to nutrient limitation may be important in explaining the dynamics of these organisms in natural situations. Further analyses are warranted to clarify the interactions between nutrient limitation and energy limitation among zooplankton. Keywords: Anuraeopsis, Daphnia, field studies, Keratella, nutrient limitation, P-limitation, phytoplankton, Rotifer Introduction Nitrogen and phosphorus concentrations are known to be highly dynamic and to potentially limit the production of aquatic ecosystems (Sterner & Hessen, 1994). Algae have relatively high ratios of C : P and C : N, and the C : N : P ratio of phytoplankton varies considerably under N or P limitation (Goldman, McCarthy & Peavy, 1979; Sterner et al., 1993). These biochemical responses of algae to resource limitation may affect their quality as food for upper trophic levels. Several laboratory studies have shown that the limitation of N and especially P reduces the quality of Correspondence: J. M. Conde-Porcuna, Department of Animal Biology and Ecology, Faculty of Sciences, University of Granada, Fuentenueva s/n, E Granada, Spain. jmconde@goliat.ugr.es algae as food for Daphnia (Groeger, Schram & Marzolf, 1991; Sterner et al., 1993; Van Donk et al., 1997; Weers & Gulati, 1997; DeMott, 1998; DeMott, Gulati & Siewertsen, 1998; Schulz & Sterner, 1999). The decrease in food quality of P-limited algae could be a direct effect of P limitation, as well as an indirect effect caused by changes in biochemistry and cell wall morphology (Van Donk & Hessen, 1993; Weers & Gulati, 1997). Urabe, Clasen & Sterner (1997) observed, under experimental conditions of very highly soluble reactive phosphorus (SRP), that Daphnia uptake of inorganic P reverses growth limitation. Boersma (2000) observed that both the mineral and biochemical limitations of phytoplankton play a role in the growth and population dynamics of Daphnia, but suggested that the mineral requirements need to be met first. Other recent experimental laboratory studies have also shown that P-limitation significantly reduces the growth rates of the rotifers Brachionus and Anuraeopsis Ó 2002 Blackwell Science Ltd 1463

2 1464 J.M. Conde-Porcuna et al. (Rothhaupt, 1995; Conde-Porcuna, 2000). Moreover, Conde-Porcuna (2000) showed that A. fissa was more sensitive than Daphnia to P-limitation and that the effect of nutrient limitation on this rotifer was stronger than that of competition from Daphnia in a laboratory experiment. These results suggest that low food quality may reduce the potential for competition between zooplankton species. Because the ratio of dissolved inorganic N to SRP (DIN : SRP), a ratio of nutrient supply, has been used as an indicator of N or P limitation for phytoplankton (Rhee, 1978; Morris & Lewis, 1988; Morales-Baquero et al., 1999), a negative relationship between this ratio and zooplankton biomass or density could be expected in natural systems. Furthermore, this negative relationship should be stronger for some rotifer species than for Daphnia because, as previously commented, some rotifers seem to be more susceptible than Daphnia to P limitation. Numerous field studies have analysed the effects of nutrient limitation and N : P ratios on phytoplankton and bacterioplankton populations in natural situations (Elser et al., 1995; Pérez-Martínez & Cruz-Pizarro, 1995; Morales-Baquero et al., 1999; Vrede et al., 1999; Yurkovskis, Kostrichkina & Ikauniece, 1999; Gonzalez 2000). Furthermore, the influence of the N : P ratio re-supplied by zooplankton on the cycling of N and P in aquatic systems has also been investigated (Sterner, 1990; Urabe, Nakanishi & Kawabata, 1995; Carrillo, Reche & Cruz-Pizarro, 1996). However, studies on the impact of nutrient limitation on zooplankton community structure in natural situations are very scarce (Hessen, 1992; DeMott & Gulati, 1999). In field studies, zooplankton growth and abundance have been traditionally analysed as a function of food concentration, taxonomic composition of the phytoplankton and abiotic factors such as temperature (McCauley & Kalff, 1981; Pace, 1984; Hewitt & George, 1987; Lampert & Rothhaupt, 1991; Conde- Porcuna, Morales-Baquero & Cruz-Pizarro, 1994; Morales-Baquero, Conde-Porcuna & Cruz-Pizarro, 1994; Fussmann, 1996; Conde-Porcuna & Declerck, 1998). However, field data on zooplankton taxa would be expected to show differences in their susceptibility to P limitation. Morales-Baquero & Conde-Porcuna (2000) observed that rotifers were the only zooplankton group in some oligotrophic lakes that seemed to be susceptible to P-limitation. DeMott & Gulati (1999) found negative correlation between seston C : P ratios and Daphnia abundance in some lakes, which suggested that high seston C : P ratios constrained Daphnia abundance in these lakes. In contrast, they observed little relationship between seston C : P ratios and other crustacean species. However, no field study has yet analysed the susceptibility of different rotifer taxa to P limitation. The present study investigated the impact of nutrient limitation, especially SRP availability (SRP concentration and DIN : SRP ratio), on the zooplankton community structure in a mesotrophic reservoir during a year-round study. Food concentration, phytoplankton taxonomy, temperature and hydraulic regime were also considered. Methods Study site Bermejales reservoir is a medium-size (562 ha) reservoir lying on the limestone located in an open valley area of the South of Spain. The reservoir is mesotrophic, with Secchi disc transparencies of 1 3 m and mean total phosphorus concentration of 18.3 lg L 1. According to Pérez-Martínez & Cruz-Pizarro (1995), Bermejales reservoir is a P-limited system with a high N concentration throughout the year. Other limnological characteristics of the Bermejales reservoir are described in Morales-Baquero et al. (1994) and Pérez-Martínez & Sánchez-Castillo (2002). Sampling regime Samples for measurements of chlorophyll a concentration (Chla) and nutrient concentration and density of Phytoplankton and Zooplankton were collected monthly from April 1988 to March These samples were taken from five depths at the deepest point of the reservoir (Maximum depth:»19 M), using an 8-L Van Dorn Sampler. Water column mean values were used for analyses. Temperature was measured on each sampling day using a YSI probe (Hydrolab 4041) (Yellow Springs Instruments, Yellow Springs, OH, USA). Dissolved nutrient concentrations were analysed after filtering water through a Whatmann GF/C filter (Whatman International Ltd., Maidstone, UK). We used the following analytical methods: Murphy & Riley (1962) for SRP, the nesslerization method for ammonium

3 Nutrient limitation of zooplankton in a reservoir 1465 (APHA, 1985), Shinn (1941) for nitrite, and the brucine method for nitrate (APHA, 1985). In the present paper, the sum of the ammonium, nitrite and nitrate concentrations will be considered as DIN. Total suspended solids were measured according to APHA (1985). Food availability for zooplankton was estimated as the Chla concentration of the edible algae fraction (< 40 lm). Chlorophyll-a samples were obtained by filtering water through a 40-lm mesh screen. They were analysed spectrophotometrically on methanolic extracts, and Chla was estimated following Jones (1979). Zooplankton samples (8 L of water filtered onto a 40-lm mesh screen) were preserved in 4% formaldehyde. Zooplankton species were identified and counted under an inverted microscope. Measurements of individuals in each sample (whenever possible, 20 animals per species) were made using a calibrated ocular micrometer. Zooplankton abundances were converted to biomass using the values and formulas given in Bottrell et al. (1976). Phytoplankton samples (100 ml) were immediately fixed in acetic Lugol s. These samples were also counted and analysed for species identification using an inverted microscope. Thirty cells of the most common phytoplankton species and between one and thirty cells of the rest of the species were measured with an ocular micrometer for every sampling day. Biomass of each individual taxon was determined by multiplying mean cell biomass (considering 1 lm 3 = 10 6 lg fresh weight) by cell population density. The water residence time (hydraulic regime) was estimated monthly as the ratio of water inflow to mean water volume in the reservoir. The absolute population changes of each zooplankton species, temperature changes and concentration changes of Chla and SRP were calculated as the difference between each sampling interval. Statistical analyses were performed using the STATISTICA program (Statsoft, Inc. 1998). Pearson correlations were used and probability values were adjusted for the number of simultaneous tests using the sequential Bonferroni method (Rice, 1989). Multiple regression analyses were performed to compensate for covariance, using the forward stepwise selection procedure to select those variables with a significant F-value that significantly increase the regression sum of the squares. Results The zooplankton community of the Bermejales reservoir was mainly dominated by cladocerans in terms of biomass, although rotifers were relatively important during autumn and winter (Morales-Baquero et al., 1994). The main zooplankton species were the rotifer Keratella cochlearis Gosse, the cladocerans Daphnia longispina O.F. Müller and Ceriodaphnia spp., and the copepod Tropocyclops prasinus Fisher. The biomass of these species was high during autumn and winter, when water residence time was low (Fig. 1). Other important zooplankton species were the rotifers A. fissa Gosse (mean biomass: 2.13 lg L 1 ) and Synchaeta oblonga Ehrenberg (mean biomass: 1.90 lg L 1 ). The phytoplankton community was dominated by dinoflagellates and diatoms, especially Ceratium hirundinella O.F. Müller and Cyclotella ocellata Pantocsek (Pérez-Martínez & Cruz-Pizarro, 1995). Regression analyses were performed using the zooplankton populations as dependent variables. Because the effects of food availability, SRP, DIN, temperature and water residence time on zooplankton biomass may be not independent, a multiple forward stepwise regression was undertaken to compensate for covariance. The result showed that SRP was the only significant variable related to total rotifer biomass (Table 1). In relation to this, Fig. 2 depicts the temporal pattern of total rotifer biomass, SRP and Chla. Total rotifer biomass showed the same temporal pattern as SRP, whereas Chla showed a different pattern. Cladoceran and copepod biomasses were not significantly correlated with SRP concentration or Chla (P > 0.05). Table 1 also shows that SRP was the most significant variable (highest partial correlation) related to the biomass of Keratella and was the only significant variable related to that of Anuraeopsis. No significant relationships were obtained between SRP and densities of S. oblonga. For crustacean species, neither the biomass of D. longispina nor the biomass of Tropocyclops prasinus was correlated with SRP, whereas Ceriodaphnia biomass was positively correlated with SRP (Table 1). The Chla was significantly related to the biomass of Synchaeta and Daphnia. Water residence time showed high and significant partial correlations with Keratella and Daphnia. When all these analyses were repeated using total edible phytoplankton biomass (< 40 lm) instead of Chla as food source for zooplankton, no

4 1466 J.M. Conde-Porcuna et al. Fig. 1 Population biomass of the main zooplankton species and water residence time (Tw) during in Bermejales reservoir. Table 1 Results of forward stepwise regression analyses (n = 12). Dependent variables: total rotifer biomass, biomass of rotifer species and biomass of crustacean species. Independent variables: chlorophyll-a content (< 40 lm), SRP and DIN concentrations, water residence time (Tw), and temperature. Only those independent variables with a significant F-value were selected in the regressions. When two variables were included in the regression, the partial correlation of each variable is shown Rotifers Keratella Anuraeopsis Synchaeta Daphnia Ceriodaphnia Tropocyclops SRP 0.90*** 0.80** 0.69* 0.67* DIN Chla 0.75** 0.87*** 0.64* Tw 0.66* 0.93*** Temperature Adj. R ns = not significant; *P < 0.05; **P < 0.01; ***P < R 2 = square multiple correlation coefficient. significant effects of phytoplankton biomass were observed. In order to analyse species-specific phytoplankton effects on zooplankton taxa, we repeated the previous regression analyses using the specific biomass of the dominant edible algae species (Table 2). For Keratella and Anuraeopsis, the results were similar to those obtained in Table 1. For Synchaeta and Daphnia, Oocystis spp. was the only phytoplankton taxon included in the regression. Oocystis spp. was the only significant variable related to Synchaeta, whereas Oocystis and water residence time were related to Daphnia (Table 2). No significant relationships were obtained between cladoceran and rotifer species and the biomass of other edible algae, which are normally good food sources for zooplankton (e.g. Dictyosphaerium, Chrysochromulina, Rhodomonas and Cryptomonas). Chrysochromulina and Oocystis were related to T. prasinus biomass but no other variable was significantly related to this copepod (Table 2). The alga

5 Nutrient limitation of zooplankton in a reservoir 1467 Fig. 2 Total rotifer biomass, chlorophyll-a concentration (Chla) and soluble reactive phosphorus (SRP) during in Bermejales reservoir. Table 2 Results of forward stepwise regression analyses (n = 12). Dependent variables: total rotifer biomass, biomass of rotifer species and biomass of crustacean species. Independent variables: species-specific phytoplankton biomass (< 40 lm), SRP and DIN concentrations, water residence time (Tw) and temperature. Only those independent variables with a significant F-value were selected in the regressions. When two variables were included in the regression, the partial correlation of each variable is shown Rotifers Keratella Anuraeopsis Synchaeta Daphnia Ceriodaphnia Tropocylops SRP 0.90*** 0.80** 0.69* DIN Cyclotella Oocystis 0.77** 0.72* 0.69* Dictyosphaerium Cryptomonas Rhodomonas Chrysochromulina 0.74** Tw 0.66* 0.87*** Temperature Adj. R ns = not significant; *P < 0.05; **P < 0.01; ***P < R 2 = square multiple correlation coefficient. Ceratium hirundinella, which is not edible to zooplankton, was significantly correlated with zooplankton biomass (r = 0.89; P < 0.001). We also analysed the relationships between the DIN : SRP ratio and the biomass of the main zooplankton groups (rotifers, cladocerans and copepods). Because SRP concentration was below detectable levels from June to August, we considered the SRP concentration during these months as 0.1 lg L 1 for the calculation of the DIN : SRP ratio. The results revealed that rotifers had a highly significant and negative correlation with the DIN : SRP ratio (Fig. 3).

6 1468 J.M. Conde-Porcuna et al. Fig. 3 Relationship between total rotifer biomass and the ratio DIN : SRP in Bermejales reservoir (r = 0.88; P < 0.001). Both variables were logarithmic transformed. Neither cladocerans nor copepods were significantly correlated with this ratio (r = 0.43 and r = 0.47 for cladocerans and copepods, respectively, P > 0.05). Similar results were obtained when these regression analyses were run excluding June August data, because of the undetectable SRP concentrations (rotifers: r = 0.78, P < 0.05; Cladocerans: r = 0.37, P > 0.05; copepods: r = 0.48, P > 0.05). In addition to these analyses, to test whether the fecundity (number of eggs per female) of rotifers could be related with P limitation, correlation analyses were performed. In the Pearson correlation analyses, fecundity of A. fissa was significantly and inversely related to the DIN : SRP ratio (Fig. 4), whereas fecundity of K. cochlearis was not (r = 0.34; P > 0.05). Multiple regression analyses were also performed between the number of eggs per female of rotifer species (K. cochlearis and A. fissa) and food availability, DIN : SRP ratios, temperature and water residence time. These results showed that DIN : SRP ratios and water residence time were now significantly correlated with the fecundity of K. cochlearis (partial correlation for DIN : SRP: r = 0.77, P < 0.01; partial correlation for water residence time r = 0.85, P < 0.01). The DIN : SRP was the only significant variable related to the fecundity of A. fissa. The fecundity of S. oblonga was not analysed because Synchaeta releases its eggs directly into the water and they sink to deeper waters where there are different conditions (low concentrations of food and low temperature) and copepod predation can produce egg mortality, decreasing population fecundity (Stemberger & Gilbert, 1987; Roche, 1990). Pearson correlations showed that population changes of Keratella and Anuraeopsis were correlated with changes in concentration of SRP (Table 3). In contrast, Synchaeta population change was correlated with changes in chlorophyll-a content (Table 3). Because Daphnia may suppress rotifers through competition in Bermejales reservoir (Conde-Porcuna et al., 1994), Daphnia population change was included in these correlations. However, no correlation was observed between Daphnia and any rotifer taxa. Figure 5 shows that the relationship between Keratella population change and SRP concentration change exhibited a higher slope than that between Anuraeopsis and SRP. This could mean that for similar changes in the SRP content, the increase in Keratella density would be higher than that in Anuraeopsis. Table 3 Correlations of rotifer species population changes with changes in temperature, SRP, edible algae and Daphnia biomass between sampling intervals (D). Edible algae (<40 lm) were correlated using biomass estimates of algae (bio) and chlorophyll-a concentrations (Chla) D Keratella D Anuraeopsis D Synchaeta Fig. 4 Relationship between the fecundity of Anuraeopsis (number of eggs per female) and the ratio DIN : SRP in Bermejales reservoir (r = 0.67; P < 0.05). Both variables were logarithmic transformed. Analysis was restricted to occasions when Anuraeopsis density was > 1 ind L 1. D Temperature 0.38 ns 0.36 ns 0.13 ns D SRP 0.86*** 0.75** 0.39 ns D edible algae (bio) 0.61(*) 0.55 ns 0.28 ns D edible algae (Chla) 0.28 ns 0.15 ns 0.80** D Daphnia 0.72* 0.59 ns 0.04 ns ns = not significant; *P < 0.05; **P < 0.01; ***P < 0.001; (*) = not significant differences using the Bonferroni correction (only one estimate of edible algae, bio or Chla, was considered).

7 Nutrient limitation of zooplankton in a reservoir 1469 Fig. 5 Relationships of rotifer species population changes with changes in SRP concentration between sampling intervals in Bermejales reservoir. Discussion The correlation analysis suggests that rotifers were affected by P limitation in Bermejales reservoir, whereas food availability had a minor effect. This P limitation had no apparent effect on cladocerans and crustaceans. DeMott & Gulati (1999) suggest that Daphnia abundance in several lakes was strongly constrained by the seston C : P ratio, whereas other cladoceran species were less sensitive to P limitation, as predicted by stoichiometry theory. However, no previous studies have shown the impact of nutrient limitation on rotifer populations in natural ecosystems. Morales-Baquero & Conde-Porcuna (2000) presented some evidence to support a greater susceptibility of rotifers to P limitation in oligotrophic lakes compared with crustaceans, but rotifer taxa, reproductive rates, food availability and seasonal changes were not considered. Several laboratory studies have shown that P-limited algae are poor food for Daphnia (Sommer, 1992; Sterner & Smith, 1993; Sterner et al., 1993; Van Donk & Hessen, 1993; Weers & Gulati, 1997) and for some rotifer species (Rothhaupt, 1995; Conde-Porcuna, 2000). Changes of the cell wall in P-limited algae have been related to low digestibility of algae by zooplankton (Van Donk & Hessen, 1993; Van Donk et al., 1997). Furthermore, nutrient limitation of algae has been related to fatty acid (Harrison, Thompson & Calderwood, 1990) and mineral limitations (Boersma, 2000). However, P limitation and fatty acid limitation are not mutually exclusive alternatives (Gulati & DeMott, 1997; Boersma, 2000). Direct mineral nutrient limitation of the rotifers caused by unfavourable cell stoichiometries of the food algae cannot be ruled out. Pérez-Martínez & Cruz-Pizarro (1995) observed in field experiments that P is the limiting nutrient for phytoplankton in Bermejales reservoir. In the present study, two rotifer species seemed to be more susceptible than crustaceans to P limitation. Our results show that the fecundity of rotifer species may be affected by P limitation. These findings are consistent with a previous laboratory study (Conde-Porcuna, 2000). The absence of a clear effect of food availability on rotifers suggests that nutrient limitation may have a greater effect than food concentration itself has on rotifer dynamics in natural systems, although rotifers may specialise in feeding on specific food classes (Edmonson, 1965). However, field studies on rotifer dynamics have not taken into account nutrient concentrations or N : P ratios. Rotifer population dynamics are usually explained by exploitative competition, interference competition and predation (Gilbert, 1988; May & Jones, 1989; Lampert & Rothhaupt, 1991; Conde-Porcuna et al., 1994; Fussmann, 1996; Conde- Porcuna, 1998). Conde-Porcuna (2000) showed that food quality may influence the strength, impact and outcome of competition and that low food quality reduces the effect of Daphnia competition. In Bermejales reservoir, there was no detectable negative relationship between Daphnia and rotifers during the present study, although other studies have shown that Daphnia suppress rotifer populations in this reservoir (Conde-Porcuna et al., 1994). However, nutrient conditions in the latter study were not considered and may have differed from those observed in the present study. High P concentration in zooplankton tissues is associated with high growth through high concentrations of RNA (Elser et al., 1996; Main, Dobberfuhl & Elser, 1997). Elser et al. (1996) showed that there can be strong ontogenetic shifts in body stoichiometry. Daphnids appear to maintain high concentrations of RNA throughout ontogeny, resulting in generally low N : P ratios at all life stages. In rotifers, the N : P ratios are not well known, although Rothhaupt (1995) showed that Brachionus has high N : P ratios. Rotifers, in contrast to crustacean zooplankton, have a fixed number of nuclei after embryogenesis and this number is not subsequently altered (eutely). Morales-Baquero & Conde-Porcuna (2000) suggested that P limitation in the environment could be mainly

8 1470 J.M. Conde-Porcuna et al. reflected in the numbers of eutelic organisms (rotifers) because they largely allocate resources to reproduction. Our results also suggest that Keratella is more susceptible than Anuraeopsis to P limitation (Fig. 5). Moreover, Synchaeta did not show any significant relationship with either SRP concentration or DIN : SRP ratio. Schulz & Sterner (1999) showed that the body of Bosmina has a lower percentage of P than does that of Daphnia and suggested that Bosmina has lower P requirements than Daphnia and is better able to grow with a low P diet. Indeed, the different susceptibility that rotifer species seem to show to P limitation in our study could be related to differences in body P content. The outcome of competition between zooplankton populations may shift in favour of zooplankton species with low P requirements under conditions of seston P mineral limitation. However, the susceptibility of various rotifers to P limitation needs to be tested. As an alternative to P limitation, it can be suggested that high SRP favours specific resources that are better used by rotifers. This may be related to changes in the phytoplankton community and bacteria abundance. However, C. hirundinella was the only algae species that showed a positive relationship with SRP (r = 0.69; P < 0.05), and C. hirundinella is not edible to zooplankton. Oocystis and Crysochromulina were the only edible phytoplankton taxa that showed a positive and significant effect on some zooplankton species. Keratella and Anuraeopsis showed no relationship with any edible phytoplankton species in multiple regression analyses. Several surveys have shown that K. cochlearis and A. fissa also feed on bacteria and detritus (Pourriot, 1977; Starkweather & Bogdan, 1980; Ooms-Wilms, 1991). This could partly explain the lack of significant relationships between phytoplankton and Keratella and Anuraeopsis in our study. Moreover, some studies have shown that bacterial production increases in response to nutrient availability (Pace & Cole, 1996) and that bacteria are one of the major regenerators of dissolved P in plankton in lakes (Hudson & Taylor, 1996). Bacteria could have higher P contents than algae in Bermejales reservoir and, because some rotifers may also feed on bacteria, they would be less affected by P limitation. Moreover, in our study, high SRP availability could also be related to high bacterial production and abundance and consequently to increased bacterial food sources for these rotifer species. Nevertheless, a previous study indicated that phytoplankton are an important food source for rotifers in Bermejales reservoir (Conde-Porcuna et al., 1994). In the present study, edible algae did not increase when SRP concentrations were high. This could be the result of high grazing pressure by zooplankton that suppressed the edible phytoplankton biomass. However, the lack of clear negative relationships between zooplankton and phytoplankton in the edible size fraction suggests that phytoplankton were not strongly or consistently affected by grazing. Some algae species have very high rates of cell division (Sommer & Kilham, 1985) that may explain the lack of a significant response despite grazing pressure, as observed by Pérez-Martínez & Cruz-Pizarro (1995) in this reservoir. Ceratium hirundinella was positively and significantly correlated with zooplankton biomass. However, this positive relationship is unlikely to be the result of a positive effect of this alga on zooplankton, because it is not edible. Previously, Pérez-Martínez & Sánchez- Castillo (2002) observed that Ceratium density was also related to the hydraulic regime of the reservoir. Our results might be explained by changes in the hydraulic regime that caused unmeasured changes in resources that favour rotifers. As high SRP values and low DIN : SRP ratios were reached after the water residence time dropped at the end of the summer, P availability may have been related to the hydraulic regime in the reservoir. Greater water inflows could increase the total suspended solids in the water of reservoirs and we found a significant positive correlation between SRP concentration and total suspended solids in Bermejales reservoir (r = 0.58; P < 0.05). This could mean that greater water inflows (short water residence time) could favour an increased SRP concentration during autumn and winter in Bermejales reservoir. However, water residence time and/or water inflow in the reservoir were not significantly associated with SRP concentration or DIN : SRP ratio (P > 0.1). Moreover, the fecundity of K. cochlearis was significantly related to the DIN : SRP ratio regardless of the hydraulic regime (measured as the water residence time in the reservoir) according to the multiple regression analysis. In summary, P limitation may play an important role in rotifer dynamics, in addition to phytoplankton availability or possible interactions with other zoo-

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